1. Field of the Invention
This invention relates to a side impact passenger protection system for a vehicle and to a crash determination device thereof.
2. Related Art
A passenger protection system for a vehicle of the related art is disclosed in for example Japanese Patent Application Laid-Open No. H.8-67231. This passenger protection system has a central acceleration sensor mounted in the middle of a car and side acceleration sensors mounted in the vicinities of center pillars in the left and right sides of the car. The central acceleration sensor detects a crash between an obstruction and the front or sides of the car. The left side acceleration sensor detects a crash between an obstruction and the left side of the car, and the right side acceleration sensor detects a crash between an obstruction and the right side of the car.
When a detection signal of the central acceleration sensor exceeds the largest of a number of threshold values, or when it exceeds a threshold value smaller than the largest of a number of threshold values and also a detection signal of the side acceleration sensor exceeds a certain threshold value, the car is determined to be in a crash and the passenger protection system is operated.
In this kind of passenger protection system, for example when an impact not necessitating operation of the passenger protection system occurs at a front wheel of the car, as happens when a front wheel mounts a curb, this impact tends to be transmitted to the central acceleration sensor earlier than to the side acceleration sensors. Consequently, the rise of the acceleration that the central acceleration sensor detects is rapid and the detected acceleration itself is also high. As a result, when threshold values of the kind mentioned above are used alone there is a possibility of the passenger protection system operating erroneously.
In this connection, the present inventors have conducted various studies into car side crash situations. Generally, at the time of a car side crash the time available for making the crash determination necessary to operate the passenger protection system is extremely short. For this reason it is necessary to employ the kind of side acceleration sensor mentioned above. However, in the side acceleration sensor, even a slight impact (an impact not necessitating operation of the passenger protection system) directly hitting the vicinity of a side acceleration sensor produces a relatively large acceleration.
On the other hand, even if an impact is one necessitating operation of the passenger protection system such as that of another car crashing into a door of the present car diagonally from the front, when that impact does not hit the side acceleration sensor directly the acceleration occurring at the initial stage of the impact is of a level below the acceleration resulting from the above-mentioned slight impact directly hitting the side acceleration sensor vicinity.
Consequently, using an acceleration caused by this kind of impact it is not possible to determine correctly within the kind of short crash determination time mentioned above that the impact is a crash of the car.
When detailed studies into this kind of phenomenon were carried out, the following results were obtained.
Because as shown in FIG. 9 and FIG. 11 each side acceleration sensor (hereinafter called the side acceleration sensor 1) is mounted in the vicinity of a center pillar in the side of the car, the side acceleration sensor 1 is disposed in a part of the car that is more rigid than other parts. It was found that in cases where this kind of mounting has been carried out, with a slight impact directly hitting the vicinity of the side acceleration sensor 1, there is almost no phase difference (see FIG. 10A) between the detected accelerations of the side acceleration sensor 1 and the central acceleration sensor (hereinafter called the central acceleration sensor 2). This is also the same in a case where another car crashes at low speed into the side of the present car as shown in FIG. 9. In FIG. 10A, P1 and R1 are graphs showing change of an integral output obtained by integrating the detection signal of a central acceleration sensor 2 and Q1 is a graph showing change of an integral output obtained by integrating the detection signal of a side acceleration sensor 1.
The rigidity of the doors of the car, on the other hand, is low compared with that of the above-mentioned center pillar vicinity. It was found that for this reason in a high-speed crash wherein another car crashes into a door of the present car diagonally from the front as shown in FIG. 11, the phase of the detected acceleration of the side acceleration sensor leads the phase of the detected acceleration of the central acceleration sensor and the phase difference between the two detected accelerations is large (see FIG. 12A). In FIG. 12A, P2 and R2 are graphs showing change of an integral output of the detection signal of a central acceleration sensor 2 and Q2 is a graph showing change of an integral output of the detection signal of a side acceleration sensor 1.